The present invention relates to imaging devices, and more particularly to imaging devices wherein a plurality of light beams are controlled by light valves.
1. Field of the Invention
Electro-optic cells are devices which can act as light valves, shutters, or modulators. Such devices are sometimes referred to as Kerr cells. They may be composed of a thin wafer of a transparent piezoelectric ceramic material, such as PLZT (lead lanthanium zirconium titanate) wherein thin electrodes are placed on one or both surfaces. The wafer is illuminated with polarized light. Subsequent to passage through the electro-optic cell the light is passed through a second polarizing filter, the axis of polarization of the second filter being perpendicular to that of the first filter. When the PLZT material is not stressed, nearly all of the light is blocked by the second polarizer or analyzer. When a voltage is imposed between two adjacent electrodes on the material's surface, the material bends slightly because of its piezoelectric nature. This bending results in a stress within the material which in turn rotates the vector of polarization of the light passing through the material. By varying the voltage between the electrodes the polarization of light passing through the material can be rotated through an entire 90.degree., and the resultant light passing through the final polarizing material can be made to go from nearly zero to some maximum.
2. Description of the Prior Art
Linear arrays of such devices have been used in various imaging devices, e.g., xerographic printers. The response time of each cell may be from slightly less than a microsecond to more than 100 microseconds, depending on the type of electrodes used and the type of driver used. Such response times are generally somewhat slower than that of a laser diode or accousto-optic modulator as might be used with a gas laser. However when a plurality of cells, e.g., 10-100 cells, is used, the overall imaging rate may be comparable to or even faster than that which is possible with conventional imaging techniques which use a single laser beam. The cost of such imaging is in some cases much lower than that which is possible with a laser. The electro-optic cells can be used with either a coherent or noncoherent light source, e.g., an ordinary incandescent lamp together with color filters, thus permitting their use in applications where the desired color of light or distribution of colors may not match that of a given laser. This is another advantage for the use of electro-optic cells.
Typically the light passed through the aforementioned assembly of light source, polarizers, and electro-optic cell linear array is focused through a lens and imaged onto a photoreceptive medium, such as a photoconductive drum or a photographic film.
One of the difficulties encountered in the prior art with such devices is that the electrode lines cast shadows which are imaged onto the photoreceptive medium. This leaves thin lines on the image, this being objectionable. This difficulty is sometimes overcome in the prior art by making the electrode lines as thin as possible, and hence below the limit of the optical system and/or the photoreceptor to resolve. This, however, makes their manufacture more difficult. In other cases in the prior art this difficulty is overcome by using very thin, transparent electrodes of evaporated metal. Such electrodes have a high electrical resistance, which then makes the response time of the cells much slower. This also is objectionable.
Another difficulty which is encountered in the prior art is that the number of cells which is desired may exceed that which can be imaged with a single chip wherein the chip contains an array of cells. Adjacent areas of image must then be aligned and discontinuities are visible if any significant mechanical or optical variations are present. These adjacent areas of image may be the result of a plurality of arrays imaging simultaneously and/or from a single array imaging at multiple points in time.